The modern scientist has many analytical tools available at his or her disposal, each of which can provide useful information about a sample, whose identities may or may not be known. Aside from using these tools to learn the identity of a sample the tools may provide information such as molecular weight of the sample, functional groups possessed by the sample, and position of the groups, all of which are valuable in determining molecular structure. To give but one of the example of how these tools can be used, they can also be used to determine sample purity, where the identity of the sample is known.
The analytical devices include mass spectrometers, UV spectrometers, fluorescence detectors, infrared spectrometers, visible light spectrometers, RAMAN spectrometers, and atomic force microscopes. Certain of these devices, such as the mass spectrometer and the RAMAN spectrometer, employ, or can employ a laser to interact with the sample under analysis.
At it simplest, mass spectroscopy is a technique that provides the measure of a mass of a molecular sample. Additionally, important structural information can be obtained about samples whose identities are unknown by measuring the masses of fragment ions produced from the sample.
In a mass spectrometer, magnetic and electric fields are used to apply force to charged particles, that is, ions, which are in a vacuum. The target sample is ionized, and the ions, which are in the gas phase, are introduced into the vacuum system of the mass spectrometer. This is easily done for gaseous or heat-volatile samples. However, many analytes decompose upon heating. These kinds of samples require either desorption or desolvation methods if they are to be analyzed by mass spectrometry. Although ionization and desorption/desolvation are usually separate processes, the term “ionization method” is commonly used to refer to both ionization and desorption (or desolvation) methods.
The choice of ionization method depends on the nature of the sample and the type of information required from the analysis. In one known technique, the molecules of interest are ionized by energetic electron collision so that the ions produced from the sample may be steered through the mass spectrometer apparatus and then detected by a detector. However, the energetic electron collision is disadvantageous in that molecules are broken apart, or “fragmented” as a result of the collision, and the resulting mass spectrum must be interpreted in the light of the resulting “fragmentation pattern” which results for the electrons. On the other hand, ionization methods such as matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) tend to produce mass spectra from involatile compounds like peptides, proteins, and DNA, with little or no fragment ion content.
When employing ESI, a sample solution is typically sprayed from a hollow needle into an orifice across a high potential difference (i.e, on the order of several kilovolts). Heat and gas flows are used to desolvate the ions existing in the sample solution droplets. Electrospray ionization can produce multiply charged ions with the number of charges tending to increase as the molecular weight increases. Electrospray ionization is effective for charged, polar and basic compounds, and permits the detection of high-mass compounds at mass-to-charge ratios (m/z) that are easily determined by most mass spectrometers (m/z typically less than 4000). It is an effective method for analyzing both singly and multiply charged compounds. The low chemical background leads to excellent detection limits, and the presence or absence of fragmentation can be controlled by adjusting the energy input into the ions.
Direct laser desorption relies on the very rapid heating of the sample or sample substrate to vaporize molecules so quickly that they do not have time to decompose. This is effective for low to medium-molecular weight compounds. Another laser desorption technique, known as matrix-assisted laser desorption ionization (MALDI), relies on the absorption of laser energy by a matrix compound. That is, the sample is dissolved in a solution containing a large molar excess of a matrix-forming material that strongly absorbs light at the laser wavelength. A small amount of this solution is placed on the laser target and dried. The matrix absorbs the energy from the laser pulse which results in the vaporization and ionization of the analyte. Again, with MALDI and ESI, little or no fragmentation occurs when the sample is ionized. This is a desirable attribute as these techniques are well suited for the analysis of relatively high molecular weight compounds, such as in the analysis of proteins.
One of the difficulties of current mass spectrometer instruments is in the translation or movement of samples in and out of the energy source that effects ionization. For example, in a laser desorption method, the laser and/or sample must move in order to scan the particular material. Such movement should be rapid while, at the same time being accurate, repeatable, and reliable. This is particularly true in the instances where a large number of samples are to be targeted, requiring considerable movement of the laser in order to transfer energy to all available samples. It is often necessary to hit each given sample about 100 times to yield good results. When, in a given session, a significant number of samples are the subject of analysis, this could mean that hundreds or thousands of hits must be achieved. It is therefore evident that a mechanism must be supplied to carry out a rapid, accurate translation of the samples across the laser beam.
Compact discs are currently used in various ways, typically as a data storage medium. There are various translation system or disc drives that have been developed at great cost for the music and computer industry to read the information from such compact discs. In general, such compact discs are a standardized size, having a diameter of about 12 cm. Add a thickness of about 1.2 mm. The compact disc is provided with a storage medium into which pits or depressions are made to produce light reflectivities that vary and correspond to digitized data. Thus, there are a number of fairly standard drives commercially available to read the discs. The drives are provided with means that move the disc through the path of a scanning laser. Typically, the disc rotates through the path at 200 to 500 rpm. Also, the drive is typically provided with means to move the laser radially with respect to the disk, across the surface of the disc, typically from the inside of the disc to the outside of the disc. The laser reads the differences in reflectivity from the pits or depressions that have been created within the recording medium that is embedded within the disc. The differences are converted into usable information, whether it be sound, text, or graphics, to name just a few of many possibilities. Present commercial disc drive systems can be used to translate the current laser diode to any position on the compact disc by automatic moving systems at the behest of an electronic input.
With present translation systems, such as those used in the MALDI mass spectrometer system, movement or translation is effected in a device capable of x-y translation wherein the device is typically moved by means of stepper motors that can move the sample or samples, or energy source, along the x and the y axes. The motion of such translation devices is indexed, that is, one step at a time and requires considerable speed to be able to move the samples quickly through the analysis process. Such translation devices and the analysis equipment using stepper motors are generally quite heavy, cumbersome and contain a considerable number of moving parts and components, any one of which can become potential source of failure.
It would be advantageous to be able to carry out a rapid scanning of a large number of samples rapidly, accurately and with great reproducibility.